The invention relates to an electric capacitor which contains a winding of paper and/or synthetic dielectric foils and metallic coatings, and having at least one winding which is disposed in a metal cup-shaped housing sealed gas tight and containing electrical supply conductors. The housing also contains an excess pressure safety system and is filled with a dielectric, halogen-free, non-polar impregnation liquid, for example, mineral oil, up to a predetermined buffer volume.
In such capacitors, the metallic coatings comprise, for example, either regeneratable, thin metalizations which are applied to dielectric foils, or to separate carrier foils, or utilizes metal foils which are wound together with the dielectric foils. A break-away protective fuse, functioning as an excess pressure safety system, usually comprises a tensioned conductor wire with a rated break point, which wire simultaneously serves as the electric supply line to the capacitor winding, and is secured, for example, to the cover the capacitor housing. In the event of an undesirable increased pressure within the interior of the capacitor housing, such pressure is transformed into an expansion force applied to crimps or corrugations formed in the capacitor housing, whereby the distance between the bottom of the cup-shaped housing and the cover thereof is increased, thus resulting in rupture of the conductor wire.
In capacitors of this type, impregnated with oil, the winding must be completely impregnated in order to suppress partial discharges therein. In addition thereto, as much as possible of the dead space present in the housing, adjacent the winding, is filled with oil so that the winding remains covered irrespective of changes in orientation or position of the capacitor. However, because of the different coefficients of expansion of the impregnation oil and the other capacitor materials, a gaseous buffer volume is necessary in capacitors employing rigid housings, in order to accommodate the relatively greater change in volume of the impregnation oil. Generally, this buffer volume is filled with air.
In a closed structure, as the oil is usually previously completely dried and de-gassed, the difficulty can arise that this air mass is absorbed by the oil and a low pressure thereby arises in the capacitor housing. In accordance with Paschen's Law, which describes a reduced puncture strength as a result of reduced pressure, ionization processes and corona discharges can therefore occur, which can openly lead to a disruptive breakdown.
In order to avoid the low pressure occurring because of the absorption of the buffer gas, such capacitors are therefore left opened for a relatively long period of time, following the impregnation, or are repeatedly aired out. However, on the other hand, corona discharges in the winding are again promoted by the air absorption of the impregnation agent. For example, given a greater cooling, the shrinking impregnation agent cannot flow sufficiently rapidly from the exterior to the interior of the winding so that gaps crack open in the winding, into which the absorbed air prefers to diffuse. With an application of an alternating voltage, corona discharges arise in these gaps in a known manner, which discharges attack the dielectric and finally lead to disruptive breakdown, and thus to the destruction of the capacitor.